1. Field of the Invention
The present invention relates to composite metal materials having a high heat crack resistance and more particularly to composite metal materials for use in a brake plate or drum of a friction brake which prevents the growth of cracks caused by heat impact or heat fatigue resulting from sliding friction.
2. Description of the Prior Art
A car is slowed or stopped by utilizing a frictional force which is produced by pressing a brake lining onto the surface of a brake plate attached to an axle. During braking, the temperature of the surface layer of the brake plate rotating at a high speed together with the adjacent portions of the axle is raised very rapidly by the frictional force between the surface layer of the brake plate and the brake lining.
Heretofore, in production of such brake plates, grey cast iron has been widely used in view of its stable coefficient of friction and excellent wear resistance. However, due to the high speed of modern cars, the conditions under which the brake plate is used have become increasingly severe. This results in the occurrence of heat cracks in the sliding friction surface of the brake plate. In many cases, the growth of such heat cracks leads to the breakdown of the brake plate.
That is, when a car is operated at high speed, the temperature of the surface layer of the brake plate due to the braking action sometimes reaches a temperature range within which softening and plastic deformation of the material constituting the surface layer occurs and, therefore, the surface layer of the brake plate is subjected to heat expansion and compression deformation due to the large compression stress applied.
When the braking action is removed after such a rapid increase in the temperature of the surface layer of the brake plate, the heat generated in the surface layer of the brake plate is dissipated resulting in the cooling of the surface layer. However, as the cooling proceeds, tensile stress is produced in the surface layer which has been compression-deformed at the high temperature under the influences of the normal inner portion of the brake plate. This tensile stress tends to cause areas of fine fracturing. On repeating the braking action, the surface layer of the brake plate is repeatedly subjected to the above-described heat cycle so that the areas of fine fractures grow to form heat cracks. Even though the tensile stress may not break the material, it remains as residual stress. After subsequent repeated heat cycles, the stress gradually accumulates finally breaking the brake plate.
When materials having high crack transmission properties are subjected to the above described heat and stress action, cracks formed therein tend to grow rapidly. Although the grey cast iron conventionally used in the production of brake plates has a stable coefficient of friction and excellent wear resistance, that is, so-called excellent brake characteristics, its tendency for crack formation and growth is greater than that of general machine construction materials. Furthermore, a tensile strength of the conventional brake plate grey cast is low.
In order to improve grey cast iron for use as a brake material, alloy elements such as Ni, Cr, and Mo have been added to the material. With such alloy element-containing grey cast iron, however, heat cracking still occurs and the growth of such cracks cannot be prevented.